Fertilization, Gametogenesis and the Effect of Androgens Paper Example

The Process of Fertilization

Fertilization is the fusion of gametes, a small motile sperm and a large nonmotile egg(ovum), resulting in the initiation of the development of a new offspring. The process of fertilization is divided into four stages; sperm preparation, sperm egg recognition and binding, sperm egg fusion, fusion of the sperm and egg pronuclei and activation of the zygote (Flesch & Gardella, 2001).

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Sperm Preparation

Sperms once ejaculated are not ready to fertilize the ovum until they undergo capacitation. Capacitation are changes that occur to the sperms once their dilution occurs in the vagina. These changes include; increased intracellular calcium, increased beat frequency of the tails and loss of surface antigens increasing the binding capacity of the sperm to the ovum (Flesch & Gardella, 2001).

Sperm-egg Binding

Sperm egg binding is aided by receptors on both the surface of the sperm and the ovum. On the surface of the ovum are zona pellucida glycoprotein III (ZPGP III) which contain receptors that bind sperms. ZPGP III binds to the galactosyl transferase enzyme on the sperm surface. The interaction of the ZPGP III and galactosyl transferase mediates the sperm egg binding (Yoshiba & Inaba, 1993).

After the sperm-egg binding, acrosome reaction occurs. The acrosome reaction is triggered by the zona pellucida. Once the sperm binds to the zona pellucida, the sperm releases its acrosome contents; acrosin and N-acetylglucoaminidase (Dor & Rudak, 1981). The role of acrosin is to enable permeability of the zona pellucida by boring holes in it. This enables the sperm to reach the egg itself. N-acetylglucoaminidase modifies the receptor binding between the egg and the sperm allowing the sperm to detach thus enabling entry into the egg.

Sperm-egg Fusion

With the acrosome reaction, the inner acrosomal membrane of the sperm is exposed. After entry into the egg, the inner acrosomal membrane fuses with the egg plasma membrane. This fusion is aided by a heterodimeric sperm membrane protein called fertilin, which has an alpha and beta subunits. The beta subunits binds to an egg integrin and fusion occurs. Once the sperm fuses, its tail stops beating and further entry into the egg is through elongation and fusion of the eggs microvili. The sperm organelles are incorporated into the cytoplasm of the ovum. The sperm nucleus, by undergoing a series of changes such as chromatin decondensation, forms the male pronucleus. The male pronucleus moves to the centre of the cell, fusing with the female pronucleus to form a diploid nucleus.

Activation

This refers to immediate events that prevents fertilization of the egg by more than one sperm, polyspermy. After the sperm-egg fusion, the cortical reaction occurs in the egg. Beneath the egg's plasma membrane are cortical granules. Once fusion occurs, the cortical granules adjacent to the site of fusion binds to the plasma membrane releasing their contents in the perivitelline space, the space between the plasma membrane and the zona pellucida. The cortical reaction is propagated over the surface of the egg by a wave of calcium. The two enzymes released into the perivitelline space are ovoperoxidase and hydrolase. Ovoperoxidase is spermicidal killing subsequent sperms. Hydrolase interferes with ZPGP III making the zona pellucida incapable of binding additional sperm.

Compare and Contrast Oogenesis and Spermatogenesis

While spermatogenesis is the production is sperms from spermatogonia, oogenesis is the production of eggs from oogonia. Spermatogenesis takes place in the testis of the male while oogenesis occurs inside the ovary in females. Moreover, all stages of oogenesis occurs in the testis while in oogenesis, all stages but the last stage occurs inside the ovary. It is important to note that spermatogenesis is a continuous process occurring from puberty and continues throughout a lifetime whereas oogenesis begins during the fetal period, continues after puberty till menopause is reached. Both spermatogenesis and oogenesis develop from the germinal epithelium, spermatogenesis from the germinal epithelium lining the seminiferous tubules while oogenesis from the germinal epithelium overlying the ovary (Grumbach & Durchame, 1960). Further, all the spermatogonia divide by meiosis, producing sperms all the time whereas some of the oogonia divide during the production of eggs , one at a time. Growth phase is short in spermatogenesis while it is prolonged in oogenesis. Upon division of the primary spermatocyte by meiosis I, two secondary spermatocytes are formed while upon division of the primary oocyte by meiosis I, a secondary oocyte and a polar body are formed. Lastly, spermatogenesis produces motile gametes while oogenesis produces gametes that are non - motile ( Dierich & Monaco, 1998).

Effects of Estrogen and Testosterone on the Body

Estrogen

In the brain, estrogen helps to maintain body temperature, may delay memory loss and regulates parts of the brain that prepare the body for sexual and reproductive development. In the ovary, it stimulates the maturation of the ovaries, and start of a woman's menstrual cycle indicating reproductive system maturation. It also stimulates the maturation of the vagina and helps maintain a lubricated and thickened vaginal lining. Moreover, it stimulates the development of the breast at puberty and prepares the glands for milk production. Estrogen also mediates the development of secondary sex characteristics in females such as growth of pubic hair, widening of the hips and increased body fat. Further, it stimulates the maturation of the uterus preparing it for possible pregnancy. Estrogen also helps in the preservation of bone density (Van Goozen & Gooren, 1994).

Testosterone

Testosterone regulates the formation and maturation of the penis and scrotum. The penis enlarges during puberty. It also stimulates prostate and genitals during puberty and also maintains the sex drive, sperm count and fertility. It stimulates the development of secondary sexual characteristics such as deepening if voice, growth of pubic hair, widening of the chest among others. Moreover, it helps to maintain healthy bones, stimulates muscle growth, induces the male typical pattern of baldness. It also helps to maintain a feeling of well being, influences cognition skills and modulates mood and behavior for example increasing aggression and reducing anxiety and depression ( Van Goozen & Gooren, 1994).

References

Flesch, F. M., & Gadella, B. M. (2000). Dynamics of the mammalian sperm plasma membrane in the process of fertilization. Biochimica et Biophysica Acta (BBA)-Reviews on Biomembranes, 1469(3), 197-235.

Yoshida, M., Inaba, K., & Morisawa, M. (1993). Sperm chemotaxis during the process of fertilization in the ascidians Ciona savignyi and Ciona intestinalis. Developmental biology, 157(2), 497-506.

Dor, J., Rudak, E., & Aitken, R. J. (1981). Antisperm antibodies: their effect on the process of fertilization studied in vitro. Fertility and sterility, 35(5), 535-541.

Dierich, A., Sairam, M. R., Monaco, L., Fimia, G. M., Gansmuller, A., LeMeur, M., & Sassone-Corsi, P. (1998). Impairing follicle-stimulating hormone (FSH) signaling in vivo: targeted disruption of the FSH receptor leads to aberrant gametogenesis and hormonal imbalance. Proceedings of the National Academy of Sciences, 95(23), 13612-13617.

Grumbach, M. M., & Ducharme, J. R. (1960). The effects of androgens on fetal sexual development: Androgen-induced female pseudohermaphrodism. Fertility and sterility, 11(2), 157-180.

Van Goozen, S. H., Cohen-Kettenis, P. T., Gooren, L. J., Frijda, N. H., & Van de Poll, N. E. (1994). Activating effects of androgens on cognitive performance: Causal evidence in a group of female-to-male transsexuals. Neuropsychologia, 32(10), 1153-1157.